EFFECT OF BAROMETER, TEMPERATURE, AND IONIZATION. 33 



16. Effect of the barometer. If we look more specifically at 

 the new data beginning with August 10, coincidences of minima and 

 maxima of the nucleation with maxima and minima of the barometric 

 pressure occur only on August 13, 25, and 27, and these are not pro- 

 nounced. In September there is no detailed similarity until September 

 1 6, but both curves have dropped somewhat toward the marked mini- 

 mum. After September 20, however, the apparent agreement of curves 

 is conspicuous up to September 24 and would be decisive if the run of 

 temperature were not similar. During the remainder of the month 

 there is no agreement rather an opposition and the two curves are 

 remarkably at variance during the unusually low barometer in the early 

 part of October. The peak of the barometric curves from October 4 

 to 8 has nothing to suggest it in the nucleation curve. We may conclude, 

 therefore, that a direct barometric effect is absent, that such coincidences 

 as seem to occur are referable to other causes, and that the method 

 used for the elimination of barometer discrepancies is to the same degree 

 vouched for. 



17. Effect of temperature. Throughout all of the observations the 

 tendency of temperature of the fog chamber to rise from morning to 

 afternoon is most probably to be regarded as the cause of a similar 

 tendency in the nucleation. There are exceptions, most of which, how- 

 ever, may be explained away. The curves show a similar general march 

 from August 10 to 23 and from here to August 29. From September 



7 to 1 8 there is much detailed agreement, as, for instance, on September 



8 to 10 and 15 to 16. The same is true after September 20, where markedly 

 coincident variation occurs. 



So in October the agreement of curves is apt to be very close, as, for 

 instance, the effect from September 30 to October 3, the general fall 

 thereafter, and the effect from October 7 to October 9. All of this will 

 appear more strikingly when the observations are averaged for several 

 consecutive days, and most of the lack of synchronism is doubtless due 

 to the difficulty of finding the true value of nucleation. 



18. Effect of ionization. To find whether there is any relation of 

 the change of nucleation in the fog chamber in the lapse of time with a 

 state of ionization of the atmosphere, measurements were made of the 

 latter quantity by Miss L. B. Joslin, using Ebert's aspirator apparatus. 

 The data are given in table 12, where V denotes the fall of potential during 

 the fiducial time of aspiration (about 10 minutes), Q the charge per cubic 

 centimeter, and n the corresponding number of ions per cubic centimeter. 

 These data are constructed in the lower curves of fig. 9, together 

 with the cotemporaneous nucleations and temperatures of the fog cham- 

 ber, on a somewhat larger scale than heretofore. It would be difficult to 



